1. Field of the Invention
The present invention relates to switching power supply devices for supplying DC stabilizing voltages. More particularly, the invention relates to switching power supply devices storing energy in the primary winding of a transformer and a capacitor during the ON-periods of a switching element and supplying the stored energy to a load from the secondary winding of the transformer during the OFF-periods of the switching element.
2. Description of the Related Art
In Japanese Patent Application No. 9-352696, there is provided a switching power supply device, in which a first switching circuit is connected to a second switching circuit on the primary winding side of a transformer T and switching control circuits alternately turn on and off switching elements included in the first and second switching circuits before and after periods during which both switching elements are turned off so that a self-excited oscillation is performed. FIG. 1 is a block diagram illustrating the switching power supply device.
In this switching power supply device, an input power source E, an inductor L, and a first switching circuit S1 are connected in series to a primary winding T1 of a transformer T. Additionally, a series circuit composed of the primary winding T1 and the inductor L is connected in parallel to a series circuit composed of a capacitor C and a second switching circuit S2. A first driving winding T3 generates a voltage substantially proportional to a voltage of the primary winding T1. The voltage of the first driving winding T3 is input to a control circuit 11. Similarly, a second driving winding T4 generates a voltage substantially proportional to a voltage of the primary winding T1. The voltage of the second driving winding T4 is input to a control circuit 12. The voltage of the control circuit 11 is input to a control terminal of a first switching element Q1 of the first switching circuit S1. The voltage of the second control circuit 12 is input to a control terminal of a second switching element Q2 of the second switching element S2. The first switching circuit S1 is formed by a parallel connection circuit composed of the first switching element Q1, a first diode D1, and a first capacitor C1. The second switching circuit S2 is formed by a parallel connection circuit composed of the second switching element Q2, a second diode D2, and a second capacitor C2.
A rectifying element Ds is connected in series to the secondary winding T2 of the transformer T. A rectifying and smoothing circuit is composed of the rectifying element Ds and a capacitor Co connected to an output of the rectifying element Ds. The rectifying element Ds is connected in parallel to a capacitor (capacitive impedance) Cs. A detection circuit 14 detecting the voltage of a load is connected between an output of the rectifying and smoothing circuit and the load. An output feedback of the detection circuit 14 is sent to the first control circuit 11.
In addition. U.S. Pat. No. 3,596,165 provides a switching power supply device, in which two switching circuits are connected to each other on the primary winding side of a transformer to perform separately-excited oscillation and a fullwave rectifier is connected to the secondary winding.
Furthermore, in Japanese Unexamined Patent Application Publication No. 5-328719 and Japanese Unexamined Patent Application Publication No. 11-136940, there are provided switching power supply devices. In each of the devices, two switching circuits are connected to each other on the primary winding of a transformer and a secondary-side winding is formed by a circuit structure as shown in FIG. 1. In this case, an inductor and a capacitor are connected in series to the primary winding. A second switching element is connected in parallel to the series circuit.
In each of the switching power supply devices above, however, there are problems as follows.
(1) U.S. Pat. No. 3,596,165
The switching power supply device is a circuit referred to as a resonant-type half-bridge circuit (ON-ON type). In this type of circuit, when each switching element is turned on. energy is transmitted from the primary winding to the secondary winding. The ON-time of each of the switching elements is substantially fixed and the switching frequency is changed to change the impedance of an LC resonant circuit connected in series to the primary winding so as to control an output power. In other words, when the LC resonant frequency and the switching frequency are close to each other, the impedance of the LC resonant circuit becomes smaller. whereby a large current flows through the transformer. so that a large output power can be obtained. In contrast, when the LC resonant frequency is far from the switching frequency, a small output power can be obtained. In such an arrangement, according to the output power, the switching frequency changes significantly. When the frequency greatly changes, the dimensions of an output smoothing circuit and a filter circuit also increase. As a result, there are problems such as interference with electronic components and increase in loss in the control circuits.
Additionally, since the switching power supply device is a separately excited oscillation type of device, the number of components increases, which hinders miniaturization of the device and cost reduction. Furthermore, in order to perform full-wave rectification, at least two diodes are required on the secondary side of the transformer.
(2) Japanese Unexamined Patent Application Publication No. 5-328719 and Japanese Unexamined Patent Application Publication No. 11-136940
Each of the switching power supply devices provided in the publications is an ON-OFF type switching power supply device in which energy is stored in the primary winding during the ON-times of switching elements and the stored energy is discharged from the secondary winding during the OFF-times of the switching elements. However, each of the devices is not a self-excited oscillation type but a separately excited oscillation type or a synchronous oscillation type. Thus, since the device requires an oscillator, a drive circuit, and the like, the number of components increases, thereby hindering miniaturization of the device and cost reduction. In Japanese Unexamined Patent Application Publication No. 5-328719, since a synchronous oscillation circuit is used, an oscillator is not required. Nevertheless, the power supply device needs an IC including a MOS-FET having high voltage breakdown properties to drive a high-side switching element. a pulse transformer for isolation and drive, and so on. As a result, in even the switching power supply device, the size of the switching control circuit and production cost increase.
(3) Japanese Patent Application No. 9-352696
The switching power supply device provided in this publication is a self-excited oscillation type, which is an ON-OFF type switching power supply device storing energy in the primary winding during the ON-time of a first switching circuit and discharging the stored energy from the secondary winding during the OFF-period of the first switching circuit. As shown in FIG. 1, since a voltage equivalent to the sum of an input voltage Vin and a capacitor voltage Vc is applied to a switching element, the switching element needs to be an element having high voltage breakdown properties. In addition, since the power supply device has a structure in which the input voltage Vin is directly applied to the primary winding T1 of a transformer T, the voltage applied to the primary winding T1 becomes higher, which hinders miniaturization of the device.
Furthermore, only excitation energy stored in the primary winding of the transformer is output to the secondary side of the transformer. The energy of the capacitor C is not output to the secondary side. As a result, a peak current value of the primary winding becomes larger, thereby increasing conduction loss.
Accordingly, it is an object of the present invention to provide a switching power supply device having high efficiency capable of reducing loss. Furthermore, in this device, stress on the switching elements can also be lowered and the size and weight of the transformer can be reduced.
In order to solve the problems described above, according to an aspect of the present invention, there is provided a switching power supply device including a first switching circuit formed by a parallel connection circuit of a first switching element Q1, a first diode D1, and a first capacitor C1, a second switching circuit formed by a parallel connection circuit of a second switching element Q2, a second diode D2, and a second capacitor C2, the first and second switching circuits forming a series circuit, an input power source connected to the series circuit, a transformer T including a primary winding and a secondary winding, the primary winding, a leakage inductor L, and a capacitor C forming a series circuit, one end of the series circuit being connected to a junction of the first switching circuit and the second switching circuit and the other end thereof being connected to the input power source, a rectifying and smoothing circuit including a rectifying element Ds. the rectifying and smoothing circuit being connected to the secondary winding of the transformer T, energy being accumulated in the primary winding and the capacitor C during an ON-period of the first switching clement Q1 and an output being obtained from the secondary winding during an OFF-period of the first switching element Q1, an ON-time of the first switching element Q1 being controlled so that an output power is controlled, a first driving winding included in the transformer T to generate a voltage substantially proportional to a voltage of the primary winding turning on the first switching element Q1, a second driving winding included in the transformer to generate a voltage substantially proportional to a voltage of the primary winding turning on the second switching element Q2, and switching control circuits alternately turning on and off the first and second switching elements Q1 and Q2 before and after periods during which the switching elements Q1 and Q2 are both turned off, the first switching element Q1 being turned on after the second switching element Q2 and the rectifying element Ds are both turned off, so that a self-excited oscillation is performed.
With the above arrangement, there can be obtained advantages as follows:
(1) Since the voltage applied to each of the first and second switching elements Q1 and Q2 is an input voltage, semiconductor elements having low voltage ratings can be used as the switching elements Q1 and Q2. For example, the on-resistance of a typical MOS-FET increases in proportion to approximately the square of the breakdown withstand voltage. However, when a switching element having a low voltage rating is used, the on-resistance becomes small, whereby conduction loss can be reduced. Additionally, usually, an element having a low voltage rating is less expensive. Thus, by reducing the heat generation of switching elements, the entire switching power supply device can have high efficiency and the device can be produced at low cost, with the weight and size reduced.
(2) The voltage applied to the primary winding of the transformer T is approximately half the voltage in the conventional switching power supply device shown in FIG. 1. As a result, the number of turns of the primary winding can be reduced and the core gap can thereby be made small. Furthermore, a transformer T having the desired voltage breakdown properties can be designed easily, whereby the transformer can be miniaturized.
(3) Since the switching elements Q1 and Q2 of the first and second switching circuits are connected in parallel to the diodes and the capacitors, the switching elements Q1 and Q2 are turned on at zero voltage, and the switching element Q2 is turned off at zero current. As a result, switching loss is greatly reduced and heat generation can be prevented.
(4) The secondary-side rectifying element Ds is turned on at zero current and the current waveform rises relatively steeply at zero level and reaches a peak point where a ratio of current changes is zero. After that, the current waveform falls at zero level again, at which the rectifying element Ds turns off. When compared with a conventional inverted triangular waveform, the waveform is like a rectangular form, whereby a peak current value can be lowered. As a result. an effective current value can be small and conduction loss can thereby be reduced.
(5) There is no need for isolation with the use of a pulse transformer or photo coupler. In this invention, the two switching elements Q1 and Q2 having different ground levels can be driven. Moreover, since the switching elements Q1 and Q2 are adapted to the self-excited oscillation structure. it is unnecessary to use a switching controlling IC with another oscillator. Accordingly, since the switching control circuits do not have complicated structures, the entire device can be made compact at low cost.
In the switching power supply device according to the invention, each of the switching control circuits may include a resistor or a delay circuit formed by a series circuit composed of a resistor and a capacitor, the resistor or the delay circuit being arranged between the first driving winding and a control terminal of the first switching element and between the secondary driving winding and a control terminal of the second switching element, respectively. In this switching power supply device, each of the first and second switching elements is turned on with a delay after the voltage substantially proportional to the voltage of the primary winding turning on each of the first and second switching elements is generated in each of the first and second driving windings.
Therefore, before and after the periods in which the two switching elements Q1 and Q2 are both turned off, the switching elements Q1 and Q2 can be alternately turned on and off easily. With this arrangement, increase in loss and destruction due to the simultaneous turn-on of the two switching elements Q1 and Q2 can be prevented.
Furthermore, in the switching power supply device, one of the switching control circuits may include a switching unit for turning off the first switching element and a time constant circuit controlling in such a manner that the first switching element is turned off by the switching unit after a predetermined period of time has passed from the generation of the voltage substantially proportional to the voltage of the primary winding turning on the first switching element in the first driving winding.
With the switching unit for turning off the first switching element Q1, the speed of switching of the switching element Q1 can be increased, whereby switching loss caused by the switching element Q1 can be reduced. In addition. with the time constant circuit setting the ON-time of the switching element Q1, the ON-time of the switching element Q1 can be arbitrarily set or controlled to stabilize an output voltage.
Furthermore, in the switching power supply device, the remaining switching control circuit may include a switching unit for turning off the second switching element and a time constant circuit controlling in such a manner that the second switching element is turned off by the switching unit after the predetermined period of time has passed from the generation of the voltage substantially proportional to the voltage of the primary winding turning on the second switching element in the second driving winding.
Similar to the previous case, the switching speed of the switching element Q2 can be increased, whereby the switching loss of the switching element Q2 can be reduced. In addition, with the time constant circuit setting the ON-time of the switching element Q2, the ON-time of the switching element Q2 can be arbitrarily set and controlled to stabilize an output voltage.
Furthermore, in the switching power supply device of the invention, the switching unit may be formed by a transistor connected to a control terminal of the first or second switching element, and the control terminal of the transistor may be connected to the time constant circuit composed of a first impedance circuit and a charge/discharge capacitor.
Therefore it is unnecessary to use a MOS-FET or an IC having a high voltage rating in order to drive the high-side switching element Q2. With the simplified structure including the transistor and the time constant circuit, the switching element Q2 can be driven. Thus, the size and weight of the switching power supply device of the invention can be reduced and the device can be produced at low cost. In addition, since it is unnecessary to use an oscillator driving each of the switching elements Q1 and Q2, further reduction in the size, weight, and cost can be achieved.
Furthermore, in the switching power supply device of the invention, the impedance of the first impedance circuit forming the time constant circuit may change according to the output power or in response to an external signal.
According to the output power or in response to signals from the outside, the impedance value of the impedance circuit forming the time constant circuit is changed. With this arrangement, the time for charging and discharging the capacitors included in the time constant circuits is changed. As a result, the ON-time of each of the switching elements Q1 and Q2 can be controlled to allow the switching elements Q and Q2 to perform switching operations in the ON-time most appropriate according to the output voltage.
Furthermore, the switching power supply device may further include a second impedance circuit including a resistor, the second impedance circuit being connected to both ends of the capacitor C or both ends of the series circuit composed of the capacitor C and the primary winding of the transformer T to apply an input voltage to the first switching circuit via the second impedance circuit.
By connecting the impedance circuit including the resistor to both ends of the capacitor C or both ends of the series circuit composed of the capacitor C and the primary winding of the transformer T, via the impedance circuit, a starting-up voltage can be applied to the first switching circuit. Without the impedance circuit, since the input voltage is applied to the capacitor C, even when a voltage is applied to the control terminal of the switching element Q1, an oscillation rarely does not start. The impedance circuit may be connected to both ends of the series circuit composed of the capacitor C and the primary winding of the transformer T. However, it is preferable that the impedance circuit is connected to both ends of the capacitor C, since this arrangement permits the voltage applied to the impedance circuit to become lower, whereby the loss can be more reduced.
Furthermore, the switching power supply device may further include a third impedance circuit including a resistor to divide the input voltage applied to the switching circuit via the second impedance circuit and apply to the control terminal of the first switching element to start a self-excited oscillation.
In this case, the voltage applied to the first switching circuit is divided by the third impedance circuit including the resistor to apply to the control terminal of the switching element Q1 so as to start a self-excited oscillation. In this situation, the voltage-dividing resistors are not connected to the input power source but connected to the first switching circuit. As a result, only when a voltage is applied to the first switching circuit, an oscillation can be started. This leads to the prevention of start-up failure.
In addition, since it is unnecessary to dispose a one-shot pulse generation circuit for starting an oscillation, the switching control circuits can be simplified. Thus, the entire device can be miniaturized and produced at low cost.
Furthermore, the switching power supply device may further include a capacitor Cs connected in parallel to the rectifying element Ds, a capacitive impedance value of the capacitor Cs being set in such a manner that when the second switching element Q2 and the rectifying element Ds arc both turned off, the capacitor Cs resonates with the inductor of the transformer T and a voltage waveform across the capacitor Cs thereby represents a waveform like a part of a sinusoidal waveform, rising at zero voltage or falling at zero voltage.
In the ON-time of the switching element Q1, charge accumulated in the capacitor (or capacitive impedance element) Cs can be output without flowing through the rectifying element Ds when the conduction of the rectifying element Ds starts. Thus, the conduction loss of the rectifying element Ds can be reduced. In addition, when the reverse recovery loss of the rectifying element Ds is reduced and a steep voltage change is controlled, noises can be decreased. Moreover, since the waveform of the current flowing through the rectifying element Ds steeply rises and the current waveform is similar to a rectangular waveform, an effective current can be reduced.
Furthermore, in this invention, the rectifying element Ds may be a switching element performing switching with a control signal.
In this case, for example, the rectifying element Ds is not formed by a typical diode but formed by a switching element such as a MOS-FET having small ON-resistance. When such a switching element performs switching with a control signal, conduction loss in the ON-time of the switching element is reduced, whereby conduction loss caused in the secondary-side rectifying circuit can be reduced.
Furthermore, in the switching power supply device of the invention, the switching element may be a field-effect transistor.
When the first or second switching element is a field-effect transistor such as a MOS-FET, the parasitic diode and the parasitic capacitor can be utilized. Thus, when the parasitic diode is used as the first or second diode D1 or D2 and the parasitic capacitor is used as the first and second capacitor C1 or C2, the diode D1 or and D2 and the capacitor C1 or C2 is not required. Accordingly, the number of components can be reduced.
Furthermore, the switching power supply device may include one of leakage inductor L and an external inductor L or the transformer T connected in series to the primary winding, in which the inductor L resonates with the capacitor C during the OFF-period of the first switching element Q1 to allow the waveform of a current flowing through the primary winding to be a part of a sinusoidal waveform.
The inductor L resonates with the capacitor C in the OFF-period of the first switching element Q1, whereby the waveform of the current flowing through the primary winding becomes a part of the sinusoidal waveform. As a result, a peak current value of the switching element Q2 and a peak value of the current flowing through the rectifying element Ds become smaller, whereby the zero current turn-off operation of the switching element Q2 can be achieved. In addition, when the leakage inductor L of the transformer T is used as the inductor L, the external inductor L is not required. Thus, the number of components can be reduced, and moreover, energy loss due to the leakage inductance of the transformer can be decreased.
Furthermore, in the switching power supply device the switching control circuit may turn on the first or second switching element after voltage across the first or second capacitor drops to zero or near zero.
The zero voltage switching operation is performed by setting a delay time in such a manner that after a voltage across the first or second capacitor drops to zero or near zero, the switching control circuit turns on the switching element Q1 or Q2. With this arrangement, the turn-on loss can be reduced and switching noises can thereby be prevented.
Furthermore, the switching control circuits may turn off the second switching element Q2 when the current flowing through the second switching element Q2 is zero or near to zero.
With this arrangement. the switching element Q2 performs the zero-current turn off operation, whereby the switching loss and switching surge occurring when the switching element is turned off can be reduced.
Furthermore, in this invention, values of the capacitor C and the inductor L may be set in such a manner that after the waveform of a current flowing through the rectifying element Ds rises from zero and reaches a peak point at which a ratio of current change is zero, the waveform again falls to the point of zero current at which the rectifying element Ds is turned off.
Since a peak current value of the current flowing through the rectifying element Ds lowers and the waveform is similar to a rectangular waveform, the effective current decreases and the conduction loss of the rectifying element Ds is thereby reduced. In addition, since the current flowing through the rectifying element Ds does not change sharply, the occurrence of switching noises is suppressed and the rectifying element Ds is turned off at zero current. whereby the reverse recovery loss is reduced.
Furthermore, the switching control circuit may control in such a manner that a ratio of an excitation quantity in a direction of the transformer to an excitation quantity in a forward direction thereof changes according to the magnitude of a load connected to an output terminal of the rectifying and smoothing circuit.
The voltage of the output of the rectifying and smoothing circuit is controlled by changing the ON-time of the switching element Q1 to supply a stabilized output voltage to the load. Additionally, for example, while making the ON-time of the switching element Q2 substantially fixed, according to the magnitude of the load connected to the output of the rectifying and smoothing circuit, the ratio of the excitation quantity in the reverse direction and the excitation quantity in the forward direction is changed. With this arrangement, changes in the switching frequency can be suppressed, whereby interference with an electronic apparatus can be prevented and loss in the control circuit can be reduced.
Furthermore, the switching control circuit may control in such a manner that the excitation quantity in the reverse direction of the transformer is zero or substantially a predetermined fixed value regardless of the magnitude of the load connected to the output terminal of the rectifying and smoothing circuit.
In this switching power supply device, by changing the ON-time of the switching element Q1 to control the output voltage of the rectifying and smoothing circuit, a stabilized output voltage can be supplied to the load. In addition, the ON-time of the switching element Q2 is controlled such that the excitation quantity in the reverse direction of the transformer is zero or is substantially a predetermined fixed value regardless of the magnitude of the load connected to the output terminal of the rectifying and smoothing circuit. With this arrangement. conduction loss in the transformer and the switching circuit due to the regeneration of a current can be reduced.
Furthermore, one of the switching control circuits may set the ON-time of the switching element to be at a minimum value or greater to perform switching even in a state in which the load connected to the output terminal of the rectifying and smoothing circuit is short-circuited.
In this case, by setting the ON-time of the switching element to be a minimum value or greater to perform switching even in the state in which the load is short-circuited, the switching operation can be continued even in the short-circuited state. Therefore, when the short-circuited state is cleared, the output is applied to the load again automatically. Thus, an overcurrent protective circuit of a self-recovery type capable of restoring output can be formed. When an ON-time equivalent to a value equal to or less than the minimum value is set, under a short-circuited load, an input voltage is applied to the capacitor C and the oscillation thereby stops, whereby a latch-type overcurrent protective circuit is formed.